Analytes in gases may be analyzed by vacuum laser photoinization and mass spectrometry. This technique is termed resonance enhanced multi-photonionization (REMPI) spectroscopy. Typically, a tunable dye laser (the pump laser) is scanned over the vibrational levels of the selected state while a second fixed wavelength laser (the probe laser) is used to induce ionization. When the pump laser is resonant, there is a great increase in ionization cross-section for a given photon flux, giving increased ion yield. The difference in ion yield between non-resonant and resonant absorption is used as a basis to record the REMPI spectrum. Once the sample is ionized, the ions are extracted to a mass spectrometer detector. By proper tuning the probe laser photoionizes the molecules of analytical interest. Analysis is performed by time-of-flight mass spectrometry with high efficiency and selectivity.
In particular, by using biological samples such as blood plasma, saliva and skin, it is feasible to detect biological analytes in such complex matrices at very low concentrations. However, detection limits using a typical REMPI system on biological analytes in such samples appear to about 0.2 to 1.0 μg/ml. Being that such biological samples are liquids, accuracy below this limit is compromised by absorption at the inlet system. Furthermore, absorption requires frequent cleaning of the system leading to slow turn around time. It would thus be desirable to avoid this problem and also to improve the inlet system such that liquid analytes may be analyzed to much lower limits of detection, such as those required for analyzing biological samples.